My brother said that the powdered moss killers work pretty well but someone has to get on the roof to apply it. When it rains it activates the powder which kills the moss. He also said that some people hire a company to do pressure washing to remove it but, if the moss has been on the roof for a long time, pressure washing it off may cause leaks or roof damage. He recommended that, once the moss is off the roof by whatever method, treating it once a year will keep it from coming back.